1. Field of the Invention
This invention pertains to electrooptical and optical devices containing low-loss compact reflective turns and to methods for making and operating such devices.
2. Description of Related Art
Electrooptic and optical devices characterized by the presence of waveguides, particularly in lithium niobate (LiNbO3), are commercially available. Electrooptic devices, such as optical modulators, have the ability to change a particular characteristic of an optical signal, such as its intensity, phase, or polarization. Electrooptic modulators, particularly LiNbO3 modulators, have application in radio frequency analog links, digital communications and electric field sensing.
The most common technique for forming waveguides in LiNbO3 substrates is by high temperature indiffusion of a titanium film which has been photolithographically defined. This technique produces high quality optical waveguides with very low propagation losses of less than 0.2 dB/cm in straight channels. The low propagation losses result, in part, from the fact that the LiNbO3 lattice is only minimally perturbed by titanium. This technique of forming waveguides produces only a small increase in refractive index, i.e., Δn of about 0.005 to 0.02, compared to that of bulk LiNbO3, which leads to a relatively weakly confined waveguide mode. While this confinement is not a problem in straight waveguides, it is a severe limitation for bent or curved waveguides.
Researchers have addressed the issue of waveguide bends from a theoretical standpoint dealing with waveguides in general, to a more empirical standpoint of dealing specifically with LiNbO3 waveguides. The optical loss or attenuation α in waveguides is a complicated function which depends on many parameters. In particular, a large radius of curvature R and a high degree of mode confinement related to a large Δn are critical factors in achieving low bend losses in curved waveguides, especially because loss depends exponentially on these two parameters. If one uses a conventional process for forming Ti:LiNbO3 waveguides, the value of Δn cannot be changed much. Thus, a low-loss bend in LiNbO3 must have a very large bend radius. A 180° semicircular turn requires a great deal of “real estate” on the wafer and is not always practical.
An example of a practical LiNbO3 electrooptic device with several LiNbO3 semicircular bends is one where several nested semicircular bends are used in a radio frequency phase shifter. The turns had a 1.2 cm minimum bending radius which produced an excess loss of less than 1 dB loss per turn. A different type of LiNbO3 electrooptic device was one where elements of a large switching matrix were interconnected by bent waveguides. In this case, the bending angle was around 3.5°, so only a small fraction of a semicircle was used. Losses of 0.9 dB/cm were achieved for a 4 cm bending radius when the waveguide was widened to increase confinement. Since only short pathlengths were used for the interconnects, the loss may have been acceptable in this application. However, it translates into a huge total loss of 11 dB for a complete semicircle with a 4 cm radius.
The need to form compact bends in an electrooptic and/or optical device is fundamentally related to the limited amount of space on the wafer. Three-inch and four-inch diameter LiNbO3 wafers are available from commercial sources, although larger sizes of optical quality LiNbO3 are not readily available. The photomask pattern is typically generated in a square which, due to practical considerations, is at most 6 cm along an edge for a three-inch wafer (that is, 6 cm is the maximum usable length) and 8 cm along an edge for a four-inch wafer. Given that the size and usable portion of LiNbO3 wafers is limited, it is therefore highly desirable to conserve space on the wafers. This permits a greater density of devices and can improve functionality. To achieve these goals, several different approaches are discussed herein for forming low-loss compact LiNbO3 turns or turns of any magnitude. These turns can be incorporated into modulators or any other electrooptic and/or optical device to increase interaction or active length and lower drive voltage, or they can be used to interconnect separate optical devices.
The first embodiment of this invention is a modification the conventional s-bend structure shown in FIG. 2 which is often used to form low-loss transitions in LiNbO3 that require small lateral displacements. As shown in FIG. 2, the s-bend waveguide 16 for single-mode signal operation includes a lower section 18 and spaced thereabove at a higher elevation is upper section 20 separated by strongly angled bend section 22. Bend section 22 connects the lower and upper sections 18,20 and is disposed at an angle from the horizontal on the x-axis, where the angle changes continuously along the s-bend waveguide 16, reaching a maximum angle θ at the mid-point of the s-bend. The vertical distance along the y-axis between the lower and upper channel sections 18,20 is h, measured from the center of the lower waveguide section 18 to the center of the upper waveguide section 20. Input light or optical input 24 is injected into the lower waveguide section, travels through waveguide 16 and exits the waveguide at 26. The distance L/2, where L is length, extends from the midpoint of the bend section 22 to the terminal portion of upper waveguide section 20 or from the end of lower waveguide section 18 to the midpoint of bend section 22.
The form of the s-bend is given by the equation (1) as y=(hx/L)−(h/2π)sin(2πx/L), where x and y represent the horizontal and vertical coordinates of a 2-dimensional figure relative to the center of input lower section 18, h is the height and L is the length. The maximum angle θ is given by equation (2) as θ=tan−1(2h/L). It has been previously shown that the excess loss in an s-bend, relative to a straight channel, is a strong function of the parameter L2/h, and that if L2/h is above a certain threshold, the excess loss accumulated in the s-bend waveguide 16 can be made negligible. Thus, for a desired bend height h, the length L must be long enough to exceed the L2/h threshold in order to achieve low losses. The actual position of this threshold depends on the waveguide confinement, which is a function of the fabrication conditions. FIG. 3 shows the measured dependence of excess loss on L2/h for a conventional s-bend, with h as a parameter, for the fabrication process used herein. These measurements were taken on z-cut Ti:LiNbO3 waveguides at the 1.55 μm wavelength using the extraordinary (TM) polarization. For L2/h of equal or greater than 150 mm, the excess loss is very small, being less than 0.2 dB. This data also shows that the excess loss is independent of h alone, at least over the region from h equal to 50 to 400 μm.
The dependence of excess bend loss on the maximum angle θ of the bend, with h as a parameter, is shown in FIG. 4 for a conventional s-bend. Although θ does become smaller as L2/h increases, there is no clear-cut dependence on loss with θ for h alone, at least up to θ of 10°. It is believed that the additional loss for a few isolated data points around 1 dB is due to imperfections in the sample edges from the edge cutting process.
The combination of a directional coupler and mirror has been previously used in a proton-exchanged LiNbO3 device, where it was incorporated at the output of a low-speed modulator to enhance system linearity. In that case, the reflective directional coupler served a completely different function than the 180° reflective directional coupler in this invention, in that it was not designed to completely transfer light from one waveguide to the other. Rather, it accepted incoming light from both top and bottom waveguides and was designed to transfer only half the power from the top waveguide to the bottom one after reflection and vice versa, acting more like a conventional y-branch.